1. Technical Field
This invention relates to a signal generator for providing a single sideband (SSB) spread spectrum signal.
2. Related Art
Currently all cellular networks use double sideband modulation to upconvert a baseband signal to a radio frequency. Hence, the same information is conveyed in both sidebands, and the signal uses twice the bandwidth than is absolutely necessary. Single sideband modulation allows the same amount of information to be transmitted using half the bandwidth of double sideband modulation, or alternatively twice the amount of information in the same bandwidth.
The next generation of cellular networks is known as Universal Mobile Telecommunications Systems (UMTS). Wideband code division multiple access (W-CDMA) will be used for 60 MHz of paired spectrum, i.e. two separate bands of 60 MHz, the lower band being used for the uplink and the higher band being used for the downlink. The use of W-CDMA facilitates high bit rates for mobile users.
The capacity of a code division multiple access (CDMA) system is determined by the number of chips per symbol (known as the processing gain) divided by the energy per bit divided by noise power spectral density (Eb/No). If the number of chips per symbol can be increased then the capacity is increased. The maximum chipping rate is limited by the available bandwidth. Single sideband modulation reduces the bandwidth required by a modulated signal by a half. Therefore if a single sideband modulated signal can be produced then either the chipping rate can be increased, or two single sideband signals (upper and lower sideband) may be employed in order to increase the capacity of a CDMA system.
However, traditional techniques used to produce a single sideband signal, such as bandpass filtering or the well known phasing method cannot be used with data where the spectrum extends down to DC.
A known method of producing a single sideband signal is shown in FIG. 1. However this complex modulator may not be used with traditional spreading codes such as PN code, Walsh codes, Gold code etc. to produce SSB because these codes are binary and do not provide a suitable complex spread spectrum signal. The autocorrelation and cross correlation properties of these signals are good. However, if the signal is transformed (eg. by the Hilbert transform) to produce a quadrature signal, then discontinuities and poor correlation properties result. Poor correlation properties result in an increase in the interference experienced by other users and thus decrease the capacity of the system. Hence, to use a modulator such as that shown in FIG. 1 a spreading code is required which has good correlation properties in both the real and imaginary domains if a corresponding increase in capacity is to be achieved.
Complex spreading codes with the desired properties are known, for example Frank-Zadoff-Chu (FZC) codes as described in “Polyphase codes with good non-periodic correlation properties”, R. L. Frank, IEEE Transactions of Information Theory, vol. IT-9, pp. 43-45, Jan. 1963. However, use of these codes produces a spread spectrum signal which is not bandlimited as will be shown later, so that whatever modulation is used the resulting signal would occupy limitless bandwidth. In “A class of bandlimited complex spreading sequences with analytic properties”, M. P. Lotter and L. P. Linde, Proc of ISSSTA 95, 22-25 Sep. 1996, it was shown that by limiting the phase shift between successive samples of the sequence to be less than π radians, a bandlimited signal may be obtained and a set of codes called analytic bandlimited complex sequences derived. The penalty paid for this filtering process is that both the autocorrelation and crosscorrelation functions of the codes are no longer ideal so the number of users which may be supported is reduced. So, although the number of chips per symbol is increased in this known system, the resulting poor correlation properties do not result in a corresponding increase in capacity.